Semiconductor wafer manufacture generally requires the performance of a plurality of processing steps according to a predetermined sequence under vacuum conditions. In one type of wafer processing system, evacuatable modules or housings connect serially, and each housing serves as the site for performing one of the processing steps, such as heating or sputter coating, or simply to initially isolate wafers from the outside environment. Wafer processing systems which utilize a plurality of serially-connected housings are commonly referred to as "in-line" processing systems. Generally, these "in-line" systems allow maximum wafer throughput and processing per unit time.
For in-line wafer processing systems where processes such as sputter deposition, vacuum evaporation, and plasma etching are performed, it is necessary to translate substrate wafer holders or pallets within the vacuum processing system to move wafers through each housing. Motion of these parts inside the vacuum is usually accomplished by using vacuum feed-throughs of various kinds. A feed-through, such as a shaft, penetrates the walls of the vacuum processing system to provide a physical connection between the atmospheric side and the vacuum side. These vacuum feed-through connections generally have seals which prevent leaks from the atmosphere into the vacuum.
Usually a rotary feed-through device is used to transmit rotational motion into the vacuum processing chamber. The rotational motion is then converted to linear motion within the vacuum chamber by means of mechanical coupling components such as a ball screw, rack and pinion, worm gear, nuts, etc. which depend on friction to operate. However, in many processing applications the cleanliness of the vacuum process system is critical. Mechanical devices which depend upon frictional forces to provide linear motion unfortunately generate particles, and hence, contamination. This, therefore, makes these devices unsuitable for applications where extreme cleanliness and particulate free operation is required in the vacuum processing chamber.
It has been proposed to use linear magnetic drives to transport wafers through various processing chambers or stations which drives utilize a continuous belt or track mounted on rotating rollers externally of the vacuum chamber. The belt includes spaced magnetic elements which move linearly as the belt rotates. A cart or wafer-carrying vehicle may be located internally of the vacuum chamber to move the wafers through the in-line processing system and the bottom surface of the cart includes magnetic elements mounted thereon. When the belt rotates, the belt magnetic elements are magnetically coupled to the cart magnetic elements, and the wafer cart follows the motion of the belt and moves linearly through successive processing stations. A problem with such transporting mechanisms, however, is that they are very complex and involve a number of moving parts which constantly require replacement or adjustment. The maintenance, in turn, is both difficult and time-consuming. Furthermore, in drive systems which utilize a continuous belt or track, the belt has a tendency to stretch after use, thus necessitating further adjustments to make sure that the belt and the cart remain magnetically coupled.
It is, therefore, an objective of the present invention to transport wafers along a wafer processing line under vacuum conditions with a minimum risk of contamination from particulates generated inside the processing chamber.
It is another objective of this invention to maximize wafer throughput for an in-line processing system and to minimize wafer handling which presents a further risk of contamination.
It is still a further objective of the present invention to transport wafers using a device which has a minimum of parts to replace or adjust, thus making the device simple and inexpensive to maintain and operate.